The electrolytic industry represented by chloroalkali electrolysis plays an important role as a material producing industry. Although chloroalkali electrolysis has such an important role, a large amount of energy is consumed in conducting chloroalkali electrolysis. Thus, in countries where the energy cost is high, such as in Japan, it is important to reduce energy consumption. For example, in chloroalkali electrolysis, for resolving environmental problems and reducing energy consumption, the electrolysis has been converted from a mercury method to an ion-exchange membrane method employing a diaphragm. After about 25 years, an energy savings of about 40% has been achieved. However, even the energy savings achieved by employing an ion-exchange membrane method is insufficient, and the cost of an electric power which is the energy required for the ion-exchange membrane method is 50% of the total production cost. However, as far as the above-described method is used, additional electric power savings is impossible. For further reducing energy consumption, a radical change such as a change in the electrode reaction must be considered. As an example, the use of a gas diffusion electrode employed in fuel cells, etc., is the means having the highest potential for and saving electric power at present.
In a conventional anodic reaction (1) using a gas-diffusion electrode in place of a metal electrode as the anode, the anodic reaction (1) below is converted to the anodic reaction (2) as follows. EQU 2NaCl+2H.sub.2 O.fwdarw.Cl.sub.2 +2NaOH+H.sub.2 E.sub.0 =2.21 V(1) EQU 2NaCl+1/2O.sub.2 +H.sub.2 O.fwdarw.Cl.sub.2 +2NaOH E.sub.0 =0.96 V(2)
That is, by converting a metal electrode to a gas-diffusion electrode, the potential is reduced from 2.21 V to 0.96 V, such that an energy savings of about 65% becomes theoretically possible. Accordingly, various investigations have been conducted for the chloroalkali electrolysis using a gas-diffusion electrode.
The gas-diffusion electrode is generally semi-hydrophobic (water-repellent) and a hydrophilic reaction layer carrying platinum, etc., on the surface thereof is connected to a hydrophobic gas-diffusion layer. Both the reaction layer and the gas-diffusion layer employ a polytetraf luoroethylene (PTFE) resin, and by utilizing the properties of the PTFE resin, both layers of the gas-diffusion electrode are formed such that a large proportion of the resin is contained in the gas-diffusion layer and the reaction layer contains a reduced proportion of the resin.
When such a gas-diffusion electrode is used for chloroalkali electrolysis, various problems occur. For example, in a high concentration aqueous caustic soda (sodium hydroxide) solution, the PTFE resin, which is a water repellent material, is liable to become hydrophilic and lose its water-repellency. To prevent the PTFE resin from losing its water repellency, a thin porous PTFE resin sheet can be applied to the foregoing gas-diffusion layer at the gas chamber side. Also, the electrolysis is carried out while supplying oxygen gas or air to the gas-diffusion electrode. However, in this case, hydrogen peroxide is partially formed as a side reaction product, and the hydrogen peroxide tends to corrode carbon which is a constituent material of the gas-diffusion electrode to form sodium carbonate. Furthermore, in an aqueous alkali solution, the foregoing sodium carbonate precipitates to sometimes clog the gas-diffusion layer and render the surface of the gas-diffusion layer hydrophilic, such that the function of the gas-diffusion electrode is deteriorated. Also, even when sodium carbonate is not formed, it is observed that by carrying a catalyst on the carbon surface, the carbon is corroded with the catalyst.
To solve the above-described problems, the selection of various kinds of carbon, the production method thereof, and control of the mixing ratio of carbon and the resin have been investigated. However, these methods cannot essentially solve the above-described problems. That is, in accordance with these methods, the corrosion of carbon can be delayed but the corrosion cannot be prevented.
Because corrosion problems do not occur when carbon is not used, various proposals have been made to use silver in place of carbon. However, a gas-diffusion electrode based on a metal is produced by a sintering method different from a gas-diffusion electrode using carbon as a constituent material, and the production method thereof is very complicated. Furthermore, it is difficult to control the respective hydrophilic and hydrophobic portions.
As a method of solving these problems and further lowering the electrolytic voltage, a method of adhering or connecting a gas-diffusion electrode to an ion-exchange membrane to substantially omit the cathode chamber, or in other words, a method of configuring the cathode chamber as a gas chamber, has been proposed. When a chloroalkali electrolysis is carried out using an electrolytic cell employing the foregoing method, caustic soda thus formed reaches the gas chamber, which is a cathode chamber, through the reaction layer and the gas-diffusion layer. Because a catholyte is not present, the foregoing method is advantageous in that it does not effect the pressure difference in the height direction of the gas chamber. Thus, when the electrolytic cell is large-sized, it is unnecessary to consider the pressure distribution. Also, the electric resistance is minimized due to the substantial absence of the catholyte, whereby the electrolytic voltage can be maintained at a minimum. On the other hand, because the permeation of caustic soda in the gas chamber direction is accelerated, the size and size distribution of the perforations in the gas-diffusion layer must be controlled. Furthermore, the caustic soda which permeates to the gas chamber side tends to clog the perforations of the gas-diffusion layer such that smooth progress of the electrolysis is hindered. This is not a problem on a laboratory scale, but in a large-sized electrolytic cell such as a practically-used electrolytic cell, the electric current distribution tends to become non-uniform due to clogging of the perforations as described above, and the electrolytic voltage is increased. That is, clogging of the perforations of the gas-diffusion layer becomes the largest obstruction for achieving large scale electrolysis.
Also, the same problems are indicated in a soda electrolysis such as a Glauber's salt electrolysis, etc., in addition to ordinary sodium chloride electrolysis.